Add audience role validation for MCP::Annotations per MCP specification

The MCP specification constrains `Annotations.audience` to an array of
`Role` values (`"user"` or `"assistant"`), but the Ruby SDK accepts any
value. The other official SDKs already enforce this (TypeScript, Python,
and PHP).

This follows the existing pattern in `MCP::Icon`, which validates its
constrained `theme` value in the constructor and raises `ArgumentError`.

Added tests in `test/mcp/annotations_test.rb`:

- valid `audience` (`["user"]`, `["assistant"]`, `["user", "assistant"]`,
  and an empty array) is accepted
- an unknown role, a mixed valid/invalid array, or a non-array `audience`
  raises `ArgumentError`

Existing tests used `["developers"]`, which is not a valid `Role`, and are
updated to `["user"]`.

The full unit suite and RuboCop pass locally.

Strictly speaking this is a breaking change: a caller that currently passes
an invalid `audience` (e.g. `["developers"]`) will now get an
`ArgumentError`. Such values already violate the MCP specification, and
`nil` / valid-role values are unaffected.

test/mcp/annotations_test.rb

@@ -5,24 +5,24 @@ require "test_helper"
 module MCP
   class AnnotationsTest < ActiveSupport::TestCase
     def test_initialization
-      annotations = Annotations.new(audience: ["developers"], priority: 0.8)
+      annotations = Annotations.new(audience: ["user"], priority: 0.8)
 
-      assert_equal(["developers"], annotations.audience)
+      assert_equal(["user"], annotations.audience)
       assert_equal(0.8, annotations.priority)
       assert_nil(annotations.last_modified)
 
-      assert_equal({ audience: ["developers"], priority: 0.8 }, annotations.to_h)
+      assert_equal({ audience: ["user"], priority: 0.8 }, annotations.to_h)
     end
 
     def test_initialization_with_all_attributes
       timestamp = Time.utc(2025, 1, 12, 15, 0, 58).iso8601
-      annotations = Annotations.new(audience: ["developers"], priority: 0.8, last_modified: timestamp)
+      annotations = Annotations.new(audience: ["user"], priority: 0.8, last_modified: timestamp)
 
-      assert_equal(["developers"], annotations.audience)
+      assert_equal(["user"], annotations.audience)
       assert_equal(0.8, annotations.priority)
       assert_equal(timestamp, annotations.last_modified)
 
-      assert_equal({ audience: ["developers"], priority: 0.8, lastModified: timestamp }, annotations.to_h)
+      assert_equal({ audience: ["user"], priority: 0.8, lastModified: timestamp }, annotations.to_h)
     end
 
     def test_initialization_by_default
@@ -36,13 +36,13 @@ module MCP
     end
 
     def test_initialization_with_partial_attributes
-      annotations = Annotations.new(audience: ["developers"])
+      annotations = Annotations.new(audience: ["user"])
 
-      assert_equal(["developers"], annotations.audience)
+      assert_equal(["user"], annotations.audience)
       assert_nil(annotations.priority)
       assert_nil(annotations.last_modified)
 
-      assert_equal({ audience: ["developers"] }, annotations.to_h)
+      assert_equal({ audience: ["user"] }, annotations.to_h)
     end
 
     def test_initialization_with_last_modified_only
@@ -81,5 +81,50 @@ module MCP
       end
       assert_equal("The value of priority must be between 0 and 1.", exception.message)
     end
+
+    def test_valid_audience_with_user
+      assert_nothing_raised do
+        Annotations.new(audience: ["user"])
+      end
+    end
+
+    def test_valid_audience_with_assistant
+      assert_nothing_raised do
+        Annotations.new(audience: ["assistant"])
+      end
+    end
+
+    def test_valid_audience_with_multiple_roles
+      assert_nothing_raised do
+        Annotations.new(audience: ["user", "assistant"])
+      end
+    end
+
+    def test_valid_audience_with_empty_array
+      assert_nothing_raised do
+        Annotations.new(audience: [])
+      end
+    end
+
+    def test_invalid_audience_with_unknown_role
+      exception = assert_raises(ArgumentError) do
+        Annotations.new(audience: ["developers"])
+      end
+      assert_equal('The value of audience must be an array of "user" or "assistant".', exception.message)
+    end
+
+    def test_invalid_audience_with_mixed_roles
+      exception = assert_raises(ArgumentError) do
+        Annotations.new(audience: ["user", "developers"])
+      end
+      assert_equal('The value of audience must be an array of "user" or "assistant".', exception.message)
+    end
+
+    def test_invalid_audience_when_not_array
+      exception = assert_raises(ArgumentError) do
+        Annotations.new(audience: "user")
+      end
+      assert_equal('The value of audience must be an array of "user" or "assistant".', exception.message)
+    end
   end
 end